In recent years, development of ML-PWBs having insulating layers and conductor layers alternately laminated on a substrate has proceeded, coping with the trend in electronic equipment towards greater compactness, lighter weight and increased mounting density.
In an ordinary ML-PWB conductor layers formed on both sides of a printed wiring board (PWB) substrate are electrically interconnected by making a through-hole piercing the substrate and plating the inner wall of the through-hole or filling the through-hole with a conductive paste. In the former case, the plated through-hole is plugged with a filling material, and a conductor layer and an insulating layer are stacked thereon to cover the exposed surface of the filling material to fabricate an ML-PWB.
Among filling materials known for this application are those comprising a bisphenol epoxy resin as a thermosetting resin, an imidazole curing agent, and inorganic particles as an additive as disclosed in JP-A-10-75027 (reference 1).
ML-PWBs having through-holes and via conductors for electrical connection of conductor layers are known. ML-PWBs of this type are produced by the steps of connecting conductor layers formed on both sides of a PWB substrate via a conductor formed on the inner wall of the through-holes (via conductor), filling the through-holes with a filling material comprising an organic polymer, forming a conductor layer by printing on the upper side of the filling material to make a connection with the exposed part of the via conductor, laminating an insulating layer on the conductor layer, laminating a conductor pattern layer and a solder resist layer thereon by printing, and providing the insulating layer with a via conductor to connect the conductor layer on the filling material and the conductor pattern layer on the insulating layer as disclosed in JP-A-6-275959 (reference 2).